Power plants, particularly those operating wet scrubbers, producing a process water must comply with strict water treatment requirements that set the treated water levels for a number of constituents, including selenium, arsenic, nitrate, and mercury. The requirements also include prohibition of treated water reuse unless the water meets set concentration limits. The U.S. Environmental Protection Agency (EPA) recently proposed rules that would require all flue gas desulfurization (FGD) process wastewaters to meet best available technology (BAT) limits prior to re-use, mixing with other plant streams, or discharge. The proposed limits are shown in Table 1.
TABLE 1MaximumAverage of daily values Constituentfor any day(30 consecutive days)Arsenic, total8 μg/L6 μg/LMercury, total242 ng/L119 ng/LSelenium, total16 μg/L10 μg/LNitrate plus nitrite, as N0.17 mg/L0.13 mg/L
EPA also proposed limits for gasification wastewater, which are shown in Table 2. For gasification wastewater treatment, the proposed limits for mercury and arsenic are much lower than the corresponding limits for FGD process wastewater treatment.
TABLE 2MaximumAverage of daily values Constituentfor any day(30 consecutive days)Arsenic, total4 μg/L—Mercury, total1.76 ng/L1.29 ng/LSelenium, total453 μg/L227 μg/LTotal Dissolved Solids38 mg/L22 mg/L
Accordingly, for a water treatment process to be commercially viable, it must consistently be able to produce treated water with concentrations that are lower than the “average of daily values” limit. While occasional “upset” conditions are allowed, the “daily maximum” limits must still be met.
Challenges
EPA's proposed effluent limit guidelines set a minimum standard of treatment that will be required to be incorporated into NPDES discharge permit revisions starting in 2017, and completing by 2021. Individual permitting authorities, however, are free to set lower discharge requirements to meet their local water quality and environmental protection requirements. As such, a few power plants may already meet or exceed the discharge requirements outlined in EPA's proposal.
For facilities that do not meet EPA's proposed standards, the challenges for implementing the changes necessary to comply with the proposed standards range from minimal to extensive. For some facilities, it may mean the addition of an extra treatment step to an existing water treatment plant or the addition of a new process with specific treatment chemistry. For other plants, it can mean an entire rethink of the water management scheme and practices.
For an affected plant, the first steps are to identify the magnitude of the challenge. Water and mass balance development will become the basis for all decisions in achieving compliance. It is important to ensure that sampling and generation of the water balance captures the variability and changes that are seen in many streams, particularly FGD blow-down, any landfill leachates, and other water that may have contacted coal combustion byproducts. Changes in fuel, air pollution control reagents, plant operation, and use of other air emissions control technologies can all change speciation of constituents and the composition of the wastewater and subsequently, the approach to treatment. As a result, the need for a robust, flexible wastewater treatment process is paramount.
Existing Treatment Technologies
FGD processes are widely used for controlling air emissions from coal fired power plants. These processes use an alkaline sorbent, usually as limestone, in a slurry form to react with flue gas constituents and produce calcium sulfite (CaSO3). The majority of these processes then use excess oxidation air to convert the CaSO3 to calcium sulfate (CaSO4) or gypsum. The gypsum can be readily separated and disposed of in a landfill, or sold.
EPA has proposed that wastewater after gypsum removal must be treated to remove a number of constituents, i.e., dissolved ions in the wastewater, to very low levels. Such constituents include selenium, mercury, arsenic, and nitrate/nitrite-nitrogen. The EPA further identified BAT for FGD streams as physical/chemical precipitation combined with anoxic/anaerobic biological treatment. GE's ABMet technology is an example of one commercially available anoxic/anaerobic biological system.
Physical/chemical treatment of wastewater is widely used at power plants. The process typically uses an alkali, e.g., hydrated lime, to raise the pH of the water to a point where metals precipitate as metal hydroxides. Additional chemicals, e.g., ferric chloride, are often added to aid in further reductions through iron co-precipitation as well as acting as a coagulant to help in settling. Depending on the loadings and targets of certain constituents, organo-sulfide chemicals can also be added to improve the removal of constituents such as mercury to low, part-per-trillion levels. Precipitated solids are removed using clarification and dewatering and then transferred to landfills. At very low levels, e.g., for mercury, process operators have had difficulty maintaining performance of the process.
Constituents such as selenium and nitrate require further treatment and anoxic/anaerobic biological treatment is usually applied. Occasionally, such processes have produced ammonia in the treated water stream which must also be removed. Systems such as GE's ABMet utilize naturally-occurring, anaerobic bacteria to create a reducing environment. These bacteria are seeded into a plug-flow biofilter that utilizes carbon media to establish a biofilm for the bacteria to attached to and grow. The water is dosed with an engineered nutrient solution, and flows through a plug-flow biofilter where it encounters different reducing conditions in series: denitrification of nitrate and nitrite to nitrogen gas; reduction of dissolved selenium to insoluble elemental particulate; reduction of mercury; and finally, sulfide generation to allow precipitation of insoluble metal-sulfides. The size of the individual reduction steps is sized according to the composition of the water to be treated, potentially reducing the ability of the process to be flexible to influent water changes. The constituents are removed from the biofilter through monthly backwashes and the solids are often sent to the physical/chemical system for co-processing with its solids. Challenges may remain for ensuring that the constituents removed from wastewater in the backwash are properly sequestered in a landfill setting.
While a number of water treatment options are available, including the options discussed above, there are a number of known issues and limitations, some of which are discussed below.
Variability in Process Performance
The concentration, concentration variability, and/or form of various constituents in the water, especially for selenium, are important factors that can affect some water treatment processes. For example, the selenate form of selenium is not well removed by physical/chemical treatment processes.
The combinations of constituents in the water can also be a factor for some processes. For example, some biological processes require the removal of nitrate before selenium can be removed. In such cases, the amount of nitrate present in the water can affect overall process performance. The amount of chloride or swings in the chloride concentration of the influent water can also change the performance of biological processes. Microbe cells are sensitive to both high and low chloride levels, and also relatively fast changes in the chloride level.
Some processes are more sensitive to the temperature of the influent water (the water can be too warm or too cold). In general, biological processes for selenium removal are more sensitive and work better in the optimum temperature range.
Variability of other factors, including oxygen reduction potential (ORP), are: little or no reduction in the concentration of some constituents, including boron, bromide, chloride and sulfate; unknown effects on water treatment process performance of coal fuel changes or coal fuel blending; high reagent usage rates; and durability for extended outage or for periods when there are frequent process starts and stops.
In addition, conventional evaporative processes produce distillate that contains higher than acceptable levels of several constituents, including selenium, mercury and chloride. Some evaporative processes also produce distillate water that has a low pH value.
Therefore, there is a desire for improved systems and methods for purifying process water. Various embodiments of the present invention address these desires.